Operation of expansion engines, such as steam turbines, and using the Organic Rankine Cycle (ORC) method for generating electrical energy by using organic media such as organic media with a low vaporization temperature, which at the same temperature generally have higher vaporizing pressures as compared with water as a working medium, is known in prior art. ORC systems represent realization of the Rankine cycle in which, for example, basically electrical energy is obtained by adiabatic and isobaric state changes of a working medium. Via vaporization, expansion and subsequent condensation of the working medium, mechanical energy is obtained and converted to electrical energy. In principle, the working medium is raised to operating pressure by a feed pump, and it is in a heat exchanger supplied with energy in the form of heat that is provided by combustion or a flow of waste heat. From the vaporizer, the working medium flows via a pressure pipe to an ORC-turbine where it is expanded to a lower pressure. Subsequently, the expanded working medium steam flows through a condenser in which there is a heat exchange between the vaporous working medium and a cooling medium, whereafter the condensed working medium is returned by a feed pump to the vaporizer in a cyclic process.
Precise monitoring and controlling of the expansion engine is essential for efficient operation and is a particular challenge depending on the working medium and its thermodynamic parameters. In this, determining the physical parameters of the live steam of the working medium supplied to the expansion engine is of particular importance. Conventionally, the live steam parameters, such as the live steam entropy and the live steam enthalpy, are determined as functions of the determined temperature and/or the determined pressure of the live steam. In ORC-systems, however, it can be advantageous with regard to their degree of efficiency, that at the beginning of the expansion of the working medium in the expansion engine, this medium is present in a two-phase state.
If the working medium in the heat exchanger is only partially vaporized, then the enthalpy can not be directly determined from the pressure and the temperature of the partially vaporized working medium because the wet steam region of the live steam enthalpy and entropy is, in addition to the pressure and/or the temperature, also dependent on the steam content.
However, the steam content can not easily be determined. If, on the other hand, the expansion engine is operated with a working medium in the supercritical region near the critical point, in the vicinity of which the density of the steam and the liquid asymptotically approach each other at the same temperature, then the live steam parameters can be determined only with great inaccuracy from the pressure and/or the temperature because the isobars at the critical point run approximately horizontally. Near the critical point, even very small changes in temperature lead to very large enthalpy and entropy changes.
There is therefore a need, and it is therefore the object of the present invention, to in a reliable manner open-loop control or closed-loop control or monitor, respectively, the open-loop controlling or closed-loop controlling of an expansion engine being acted upon by a two-phase working medium such that the above-mentioned problems can be overcome.